A tire uniformity inspection machine is used to make measurements which characterize the uniformity of construction of a tire. This is accomplished by measuring the nature of certain reaction forces generated by the tire as it rolls under load along a surface.
In a typical tire uniformity inspection machine, testing is fully automatic. Tires are fed by conveyor to a test stations where each tire is mounted upon a chuck, inflated and rotatably driven with its tread surface in forced contact with the circumferential surface of a loadwheel. The loadwheel is mounted on a spindle to rotate freely about a vertical axis lying parallel to the axis of rotation of the tire. The loadwheel spindle is instrumented with load cells which measure forces acting on the loadwheel in directions of interest. Both the loadwheel and its spindle are mounted on a slidable carriage which is driven in the radial direction by a servomechanism. The servomechanism positions the carriage according to a feedback signal derived from the instantaneous force signals generated by the load cells on the loadwheel spindle in order to reach a specified average radial load setpoint.
To make data taken from a population of tires meaningful for comparison, it is necessary to insure that each tire is tested under substantially the same conditions. For instance, before measurements can be taken, the tire under test must be loaded with a specified average radial force. Radial force is measured along the direction between the center of the tire and the closest point on the rolling surface. The instantaneous radial force indicated by the load cells varies as a periodic function with the rotational position of the tire. The term "average radial force" refers to the average of the instantaneous radial force measured over one or more complete revolutions of the tire. To minimize the length of the overall test cycle and hence, the cost of testing, it is desirable to accurately bring the tire under test to the specified average radial load as rapidly as possible.
The difficulty in arriving at the average radial load setpoint rapidly arises from the fact that the instantaneous radial force exerted by the tire upon the loadwheel varies with the rotational position of the tire. If the servomechanism which positions the loadwheel uses a feedback signal emanating directly from the loadcells, the system will tend to seek the current instantaneous radial force rather than average radial force as desired. What is required is a feedback signal correlated to the average radial load rather than the instantaneous load. However, if the feedback signal is filtered to cause the servo to respond to the average radial force, a filter having a relatively long time constant is required. The time delay introduced by the filter requires the system to be operated at low gain in order to avoid oscillation. This gain limitation in turn limits the speed with which the desired average radial load can be achieved.
Accordingly, it is an object of the present invention to provide an apparatus and method for accurately and rapidly imposing a predetermined average radial load force on a tire in a way which does not require a filter having a long time constant in the feedback loop. It is a further object of the invention to provide such an apparatus and method which operates according to a feedback signal representative of the calculated average radial load on tire at the current radial distance between the tire and the loadwheel.